The terms continuous production or ‘endless rolling’ are used when a casting plant is connected to a rolling plant in such a way that the strand which has been cast in a die is guided directly—without separation from the strand part that is currently being cast and without intermediate storage—into a rolling plant where it is rolled to a desired final thickness. The start of the strand can therefore already be finish-rolled to form a steel strip having the final thickness while the casting plant continues to cast onto the same strand, such that the strand actually has no end. This is also referred to as direct-coupled operation or endless operation of the casting and rolling plant.
In the case of semicontinuous production or ‘semicontinuous rolling’, the cast strands are divided off after casting and the divided strands or slabs are supplied to the rolling plant without intermediate storage or cooling to ambient temperature.
The strand emerging from the casting plant first passes through a strand-guiding device that directly adjoins the die. The strand-guiding device, which is also called a ‘strand-guiding corset’, comprises a plurality (usually three to six) of guide segments, each guide segment comprising one or more (usually three to ten) pairs of guide elements that are preferably embodied as strand support rollers. The support rollers can rotate about an axis running orthogonally relative to the transport direction of the strand.
Individual guide elements can also be embodied as static e.g. runner-type components instead of strand support rollers.
Irrespective of the actual embodiment of the guide elements, these are disposed on both sides of the strand surfaces, such that the strand is guided by upper and lower series of guide elements and is conveyed to a roughing train.
To be precise, the strand is not supported solely by the strand-guiding device, but is also already supported by a lower exit region of the die, which can therefore also be considered part of the strand-guiding device.
The strand solidification starts at the upper end of the (open-ended) die, at the bath level or so-called ‘meniscus’, said die being typically 1 m long (0.3-1.5 m).
The strand emerges vertically downwards from the die and is deflected into the horizontal. The strand-guiding device therefore has a course that is essentially curved through an angular range of 90°.
The strand emerging from the strand-guiding device is reduced in thickness by the roughing train (HRM: high-reduction mill), and the resulting intermediate strip is heated by means of a heating arrangement and finish-rolled in a finishing train. Hot-rolling is performed in the finishing train, i.e. during rolling the rolled stock has a temperature which is higher that its recrystallization temperature. In the case of steel, this is the range above approximately 750° C., and hot rolling normally takes place at temperatures up to 1200° C.
When hot-rolling steel, the metal is normally in an austenitic state, in which the iron atoms are so disposed as to be cubic face centered. Rolling in an austenitic state is said to take place when both the starting temperature and the finishing temperature lie in the austenitic range of the steel concerned. The austenitic range of a steel is dependent on the steel composition, but is normally higher than 800° C.
Important parameters during the production process of hot steel strip from a combined casting and rolling plant are the casting speed at which the strand leaves the die (and passes through the strand-guiding device) and the mass throughput or volume flow, which is specified as the product of the casting speed and the thickness of the strand and is usually denoted by the unit [mm*m/min].
The steel strips thus produced undergo postprocessing for inter alia motor vehicles, household appliances and the building trade.
The continuous and semicontinuous manufacture of hot steel strips is known in the prior art. As a result of the casting plant and rolling plant being coupled together, the management of all plant parameters represents a considerable challenge in terms of process engineering. Modifications during the casting and rolling process, particularly as a result of changing the casting speed in combination with the strand thickness and changing a material-specific solidification coefficient that can be controlled by means of cooling, have a significant effect on the manufacturing quality and energy efficiency of the plant.
Methods and/or plants of the type in question are disclosed in EP 0 415 987 B1, EP 1 469 954 B1, DE 10 2007 058 709 A1 and WO 2007/086088 A1, for example.
Significant progress in the field of hot-rolling technology has been made by Acciaieria Arvedi S.p.A. in particular, which has developed a thin strand endless method that is based on ISP technology (In-line Strip Production) and goes by the name of Arvedi ESP (Endless Strip Production).
This technology allows a steel strip having a thickness of less than 0.8 mm to be manufactured without winding problems, wherein consistent and repeatable mechanical properties can be guaranteed over the entire width and length of the steel strip.
Using this ESP method, the casting and rolling operations are linked in a particularly advantageous manner, such that subsequent cold rolling is no longer required for many grades of hot steel strip. For grades of hot steel strip which still require subsequent cold rolling, the number of mill stands can be reduced in comparison with conventional rolling mills.
An ESP plant made by Arvedi for the production of hot steel strip, as published in e.g. the Rolling & Processing Conference '08 (September) and installed at Cremona (Italy), comprises a roughing train of three roughing stands adjoining a continuous casting plant, two strip separating devices, an induction furnace for the intermediate heating of the rough-rolled intermediate strip, followed by a finishing train of five finishing stands. The finished strip emerging from the roughing train is cooled in a cooling section and wound onto strip rolls weighing up to 32 tonnes by means of three underfloor coilers. A separating device in the form of a high-speed shearing machine is arranged in front of the underfloor coilers. Depending on the steel type and the thickness of rolled steel strip, the production capacity of this single-strand production line is approximately 2 million tonnes per year. This plant is also described in the following publications: Hohenbichler et al.: ‘Arvedi ESP—technology and plant design’, Millenium Steel 2010, 1 Mar. 2010, pages 82-88, London, and Siegl et al.: ‘Arvedi ESP—First Tin Slab Endless Casting and Rolling Results’, 5th European Rolling Conference, London, 23 Jun. 2009.
Such a plant allows hot strips having a final thickness of between 0.8 mm and 4 mm to be manufactured in continuous operation. For final strip thicknesses of between 4 mm and 12 mm, steel strip coils can be produced in semicontinuous operation, though according to calculations a width-specific minimum throughput of approximately 450 mm*m/min is required for low-carbon steels in continuous operation, in order to allow use of all five finishing stands in the finishing train.
Below this minimum throughput, only four finishing stands can be used, it being barely possible to achieve a volume flow of 400 mm*m/min for steel grades that must be cast more slowly due to specific requirements in terms of material properties. If faster cooling of the hot steel strip (intermediate strip) is required due to process engineering considerations, use of four finishing stands is questionable and use of only three finishing stands is indicated, even in the case of volume flows in the range of 400-450 mm*m/min.
In particular, an excessive strand support length of 17 m is disadvantageous, said length being the distance (more precisely known as the ‘metallurgic length’) between the discharge region of the die (specifically between the bath level or ‘meniscus’ of the liquid steel) and that end of the strand-guiding device facing the roughing train.
As described in the introduction, the strand-guiding device forms a partly curved receiving slot between the guide elements or strand support rollers for receiving the fresh cast strand (which still has a liquid core).
The end of the strand-guiding device is therefore understood in this context to mean the active guiding surface or surface line, which provides contact with the strand, of the last guide element (or last support roller in the upper series of guide elements) facing the roughing train.
A strand support length of 17 m results in complete solidification of the cross-sectional core of the strand before the strand emerges, and indeed several meters already before the end of the strand-guiding device. The technical processing advantage of a hot steel strip core, as per the ISP method, is therefore lost or not sufficiently utilized. The rolling of a completely solidified or cooler cast strand requires considerably greater energy expenditure than the rolling of a cast strand having a very hot cross-sectional core.
As the distance from the meniscus increases, the strand that is guided in the strand-guiding device or the steel strip in its initial form cools progressively. The inner region of the strand, which is still liquid or has a doughy/molten consistency, is subsequently referred to as a molten core. A ‘molten core tip’ of the molten core, being some distance from die, is defined as that central cross-sectional region of the strand in which the temperature only just corresponds essentially to the steel solidus temperature and then falls below this. The temperature of the molten core tip therefore corresponds to the solidus temperature of the respective steel type (typically between 1300° C. and 1535° C.).
For volume flows of less than 380-400 mm*m/min, the ISP or ESP method was previously only used for discontinuous production (‘batch mode’).
In the case of strand thicknesses of 45-65 mm, CSP (Compact Strip Production) methods described in the prior art likewise operate with volume flows below approximately 400 mm*m/min using a roller hearth furnace having a length of 250 m or more, wherein only discontinuous production (‘batch mode’) or semicontinuous production takes place. In the latter case, 3-6 separated (no longer connected to the casting plant or die) strands or slabs are endlessly rolled.
EP 0 889 762 B1 proposes a volume flow 0.487 mm2/min (converted to the customary unit cited in the introduction: 487 mm*m/min) for the endless casting and rolling of hot strip. For many steel types, however, casting with such a high volume flow and a relatively modest strand thickness proves to be too fast to allow a satisfactory manufacturing quality to be guaranteed.